dbuf command filtering does not work in all invocations (#184)

The dbuf command can select dbufs that match certain parameters (e.g.
'-l 1' for level-1 dbufs).  This works when `dbuf` is the first command
in the pipeline, and when it is used as a locator with a dnode_t* as
input.  But it doesn't work in other circumstances, for example
when it's being used only as a pretty-printer (with the list of dbufs
piped in), or when it's used in the middle of a pipeline.

This change adds the filter calls to this use cases, and adds
corresponding test cases

Author: ahrens

sdb/commands/zfs/dbuf.py

@@ -83,7 +83,7 @@ def ObjsetName(os: drgn.Object):
     def pretty_print(self, dbufs):
         print("{:>20} {:>8} {:>4} {:>8} {:>5} {}".format(
             "addr", "object", "lvl", "blkid", "holds", "os"))
-        for dbuf in dbufs:
+        for dbuf in filter(self.argfilter, dbufs):
             print("{:>20} {:>8d} {:>4d} {:>8d} {:>5d} {}".format(
                 hex(dbuf), int(dbuf.db.db_object), int(dbuf.db_level),
                 int(dbuf.db_blkid), int(dbuf.db_holds.rc_count),
@@ -115,6 +115,10 @@ def all_dnode_dbufs(self, dn: drgn.Object) -> Iterable[drgn.Object]:
     def from_dnode(self, dn: drgn.Object) -> Iterable[drgn.Object]:
         yield from filter(self.argfilter, self.all_dnode_dbufs(dn))
 
+    @sdb.InputHandler(input_type)
+    def from_dbuf(self, dbuf: drgn.Object) -> Iterable[drgn.Object]:
+        yield from filter(self.argfilter, [dbuf])
+
     @staticmethod
     def all_dbufs() -> Iterable[drgn.Object]:
         hash_map = sdb.get_object("dbuf_hash_table").address_of_()